1. Field of the Invention
The present invention relates to alignment in a projection exposure apparatus, and more particularly to baseline measurement in case of an alignment operation through a projection optical system with light of a wavelength different from that of the light for exposure.
2. Related Background Art
For alignment of a mask and a substrate in conventional projection exposure apparatus, there is already known a method of projecting light for alignment of a wavelength different from that of light for exposure onto the mask and the substrate through a projection optical system, thereby achieving relative alignment of said mask and substrate.
However, the lights of different wavelengths, such as the light for exposure and that for alignment, will have different focus positions when projected the projection optical system due to the color aberration. Consequently the exposure based on the positional information detected with the aligning light will involve a positional aberration corresponding to said color aberration.
Following methods have been proposed to avoid this drawback.
FIG. 1 illustrates an apparatus for avoiding this drawback, in which the error in focus position (or baseline) on the substrate (for example semiconductor wafer) with respect to the mask (or reticle) for the aligning light and the exposing light is measured in advance, and said error is eliminated in the alignment operation.
Referring to FIG. 1, light LA of exposing wavelength emitted from a light source 200 through an optical fiber 202 is transmitted through a lens 204, then enters and is reflected by a half mirror 206, transmitted through a lens 208, reflected by a mirror 210 and illuminates a reticle R bearing a first reticle mark RM.sub.1.
The light LA transmitted through the reticle R illuminates, through a reducing projection lens 212, a wafer W placed on a stage 214. At the side of the stage 214 there is provided an interferometer 215 for monitoring the position of the stage 214.
The stage 214 is provided, at the same height as that of the surface of the wafer W, with a fiducial mark FM is composed, for example, of a chromium pattern formed on a glass plate, and allows observation of the mark with a high contrast.
The light reflected from the wafer W (or fiducial mark FM) travels back the same path as that of the illuminating light and enters the half mirror 206. It is then transmitted through the half mirror 206 and a lens 216 and is received by a detector 218. For the light LA of exposing wavelength, the reticle R and the wafer W are mutually conjugate with respect to the projection lens 212, so that the light from the reticle R and that from the wafer W are simultaneously focused on the detector 218 and observed.
On the other hand, aligning light LB, from an unrepresented light source, of a wavelength different from that of the exposing light is transmitted through a lens 220, then reflected by a mirror 222 and illuminates a second reticle mark RM.sub.2 on the reticle R. The aligning light LB transmitted through the reticle R is transmitted by a correction lens 224 and the reducing projection lens 212, and illuminates, as a light spot, a wafer mark WM (not shown) on the wafer W.
The baseline measurement is conducted by illuminating, with the light LA of exposing wavelength, the first reticle mark RM.sub.1 on the reticle R and the fudicial mark FM through the projection lens 212. Then the stage 214 is so moved that the images of the reticle mark RM.sub.1 and the fiducial mark FM, simultaneously observed by the detector 218, mutually coincide, and the illuminating position of the light transmitted through the first reticle mark RM.sub.1 is determined by calculating the measurement with the interferometer 215 or the signal from the detector 218.
Then the reticle R and the fiducial mark FM are illuminated by the aligning light LB in the same manner as with the exposing light LA, and the stage 214 is moved to align the second reticle mark RM.sub.2 with the fiducial mark FM. Subsequently the illuminating position of the light transmitted through the second reticle mark RM.sub.2 is measured with the interferometer 215.
The distance (baseline) between the illuminating position of the light LA of exposing wavelength and that (focus position) of the aligning light LB on the wafer W with respect to the reticle R is measured by the interferometer 215, from two positional information detected as explained above.
Subsequently the alignment operation is conducted with the aligning light LB, with the correction of distance of the light LA of exposing wavelength and the aligning light LB on the wafer obtained above.
In the above-explained conventional baseline measuring technology, the precision of measurement is inevitably deteriorated because of the use of different marks (first reticle mark RM.sub.1 and second reticle mark RM.sub.2), because the measurements with different wavelengths are conducted at different timings, and because the hardware sequence is different for the aligning systems of both wavelengths.
Such deterioration in the precision of baseline measurement results in an offset error giving rise to the deterioration of precision of alignment itself.